Welcome! Sounds like a great history already with Toyota. Those little Tercels ran (still do run in many cases) forever...great car! We look forward to your contributions to our site!
Incidentally, the GS is worth every penny...I'm on my third one, and they are fabulous cars.
Here's your GS/ES/Avalon differences:
GS: elegant, sporty, absolutely silent interior, more of a personal driver's car, sleek exterior/interior, usually RWD, sometimes AWD. There's a lot less of these around than the other two, so more exclusivity.
ES: smooth, quiet, comfortable, gorgeous interior, softer ride, beautiful woods/chrome/leathers, FWD. Both the ES and the Avalon are larger, especially in the back seat, than the GS.
Avalon: also smooth, quiet, and comfortable, FWD, even softer ride than the ES, but lacking in fit, finish, amenities, and upscale materials compared to the Lexus. A big comfortable car, and a great buy...but not quite a Lexus. You also have one less year on the warranty than the Lexus and usually no service loaners at the dealership unless you have an undriveable/unsafe vehicle.